This invention relates to the transformation toughening of zirconia and other ceramic alloys by utilizing one or more stabilizer additives and a toughening agent. It focuses primarily on the use of novel toughening agents in making a final product with high toughness.
Transformation toughening is associated with the volume change that accompanies the tetragonal-to-monoclinic phase transformation. This transformation can be controlled by incorporating one or more stabilizing oxides into the ceramic matrix material, resulting in retention of the tetragonal phase from high temperature down to room temperature. Transformation toughened, tetragonal, partially stabilized zirconia and ceramic matrix materials toughened by tetragonal, partially stabilized zirconia have proven useful in areas where excellent thermal conductivity, hardness, toughness and strength are required, namely, wear/abrasion resistant ceramics, thermal shock resistant ceramics, cutting tools, draw dies, ceramic bearings, and oxygen ion conductors.
U.S. application Ser. No. 926,655, filed Nov. 4, 1986 under the title TOUGHENED ZIRCONIA ALLOYS, provides an extensive discussion of ceramic alloys containing zirconia and/or hafnia as the primary component, with particular emphasis being given to the mechanism involved in the transformation toughening of zirconia and/or hafnia partially stabilized with yttria through the presence of niobate and tantalate compounds. U.S. Pat. No. 4,753,903 provides further discussion of transformation toughened zirconia alloys wherein titania and yttria are included in the composition.
Thermal barrier coatings of zirconia stabilized or partially stabilized with yttria are currently utilized in gas turbine and other high temperature heat engines. These coatings are degraded relatively rapidly through the loss of the yttria stabilizer due to the formation of yttrium vanadate resulting from the presence of vanadium compound impurities in the fuels being used. The development of zirconia alloys containing a vanadium compound incorporated therein would yield bodies exhibiting much increased resistance to that source of degradation.
Heretofore, the focus of stabilizer use has been to add a particular oxide dopant to zirconia to obtain specific crystal phase structures and phase distribution, and to arrest the transformation reaction at a defined stage of its operation. As a result, although early researchers worked with such oxides as MgO and CaO for stabilizing ZrO.sub.2, more recent research has utilized exotic and costly additives in moderate concentrations such as Y.sub.2 O.sub.3, CeO.sub.2, and other rare earth oxides to produce materials demonstrating many desirable properties. Needless to say, the search for less costly stabilizing agents for ZrO.sub.2 has been continuous.